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Questions re: absorption spectrum

  1. Mar 1, 2010 #1
    Hi readers,

    I'm confused about some aspects of absorption spectroscopy, and hoping someone can de-confuse me

    First, just a preliminary question; am I right in thinking that when an electron absorbs a photon, the electron will then emit a photon of the same frequency and energy as the one it has just absorbed?

    If the answer to the above is yes, then why is it that a gas (such as hydrogen) can cause gaps (black lines) in the spectrum of an incident beam of white light? I understand that electrons within the atoms of the gas will absorb some wavelengths of the white light, but if the electron then emits a photon of the same frequency and energy, then all of the frequencies should come out of the gas intact, right?

    Is it the case that the emitted photon is sent off in a different direction, so that it doesn't end up in the spectroscope? Is that why we see the black lines in the absorption spectrum?
  2. jcsd
  3. Mar 1, 2010 #2
    Yes you've got the answer there.
    The re-emitted photons come out in all (random) directions rather than all in the original direction. (Towards you) The result is the dark line. It's not totally "black" as there are some photons emitted in your direction. Just very few.
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